1. Field of the Invention
The present invention relates to improvements in marine construction and is more specifically concerned with a curved strut for a propeller shaft and propeller designed to take advantage of strut wake, direct flow and increase propeller efficiency and reduce premature wear on the propeller shaft.
2. Description of Related Art
The performance of a ship's propeller is significantly influenced by the wake created by the movement of the ship's hull through the water. It is a consistent goal in the marine industry to improve propeller efficiency in marine vessels. This increases speed and often reduces wear. Further, with increased efficiency, a user may reduce consumption of gasoline. Struts provide stability to propeller shafts. Ship's hulls can have helically shaped channels which create a tangential direction of flow of water into the propeller.
A strut is a bracket outside the hull of a boat or ship. The strut supports the propeller shaft of a marine propeller attached to the shaft and drive system. Struts, at their junction with a marine bearing, are located at a slight angle to the axis of the propeller and vessel in order to minimize resistance and maximize efficiency of the propeller, which leaves the trailing edge of the propeller wheel race in a twisting motion similar to a helix. Certain designs relate to struts that are twisted along their longitudinal axis so that where they are connected to the bottom of the hull they are substantially in line with the axis of the vessel. At this juncture, the effect of the helical swirl from the propeller wheel race is thought to be minimal.
Several forces influence the flow pattern of water into a marine motor propeller. A determination of such flow patterns can be accomplished by analyzing flow dynamics once the water enters the propeller. By determining the flow vector, the strut can be angled such that fluid inflow is maximized. Doing so will reduce friction and resistance by the strut.
The Magnus effect is rotational flow of water around the propeller shaft while the propeller is spinning. Such Magnus effect pulls water into the strut at an angle relative to the speed of the boat and the rotational speed of the shaft. The angle at which the propeller shaft runs to the bottom of the boat is called the “shaft angle”. The angle at which the boat runs to the water is called the “running angle”. The Magnus effect and the void created aft of the shaft by the shaft angle and running angle accentuates the flow pattern and increases the angle required at the upper section of the strut leg.
Ship rudders are mounted in the stern of a ship and behind the propeller. Thus, rudders are affected by an accelerated and swirling inflow, which is also induced by the helical hull design and the propeller. Such swirled flow increases the angle at which the rudder is affected. This may cause cavitation zones which lead to erosion of the rudder surface. In addition, rudders may cause reduction in propeller performance.
Interaction between the propeller and the rudder creates an increase in turbulence and a drop in efficiency. Most rudders in the marine industry utilize symmetrical profiles having increasing thickness with a view to reduction of hydrodynamic load. An increase in performance of the ship may be achieved by aligning the rudder profiles with the direction of incident flow. However, a more practical solution is to align each rudder profile with the direction of average flow during propeller rotation.
An increase in a ship's propulsive efficiency can be created by using a profiled rudder. The profiled rudder utilizes the rotational energy created by the slipstream flow produced by the propeller. Every propeller creates vortices. Vortices are regions of low pressure, oftentimes filled with tubes of cavitation. Spirals can be formed behind the propeller. The rudder intersects the spirals to cause cavitation on the rudder.
The existing strut designs typically include a single vertical leg extending from the bottom of the hull of a boat to the propeller shaft. The typical leg is foil shaped and sits parallel to the centerline of the hull. The leg obstructs the direct flow of water towards the propellers. Additionally, typical strut designs rake aft. Typically, the strut has a barrel that houses a main bearing. Cavitation is often a result of the strut not being inline with the flow of water into the propeller. Cavitation is the formation of gas-filled cavities within liquids by mechanical forces. Cavitation can cause the paint on the bottom of the hull to wear off. This is caused by the collapse of cavitation bubbles on the surface of the metal or liquid.
Several attempts to solve such problems have been presented where the inflow of water is turned with fixed or adjustable fins or vanes to cause the water flow to counter the rotation of the propeller. Thus, this creates increased efficiency because output swirl is reduced from the propeller. The instant inventions involve a different concept and approach, by desiring to match and utilize the rotational inflow instead of creating counteracting forces.
U.S. Pat. No. 2,974,628 issued to Erlbacher on Mar. 14, 1961 entitled TWISTED STRUT CONSTRUCTION FOR MARINE BEARING WITH FORWARDLY MOUNTED PROPELLER discloses a rearwardly mounted V-shaped strut supported from the hull with the apex of the V being formed on the propeller bearing. The two separate longitudinal portions of the V-shape are mounted to the hull. The struts, at the apex, are located at a slight angle to the axis of the propeller and the boat. The two struts are twisted along their longitudinal axis and are further streamlined in cross section so as to minimize water flow. This specific twisted strut construction used to mount the propeller to the underside of a hull. This is a custom designed strut and brace assembly, in which the struts are actually placed behind the propeller blades for the specific purpose of eliminating the obstructions offered to the flow of water encountering the propeller. The struts are actually mechanically connected to the propeller bearing, and therefore the struts are behind the forwardly mounted propeller. The '628 patent teaches that the twisted relationship of the struts behind the operation of the propeller blades creates a minimum of resistance to the wheel race and to the movement of the undisturbed water past the vessel. It was also a goal of this design to provide a forwardly mounted propeller “without any strut or bearing structure in front of it”, with a supposed achieved efficiency in operation.
U.S. Pat. No. 1,990,606 issued to Junkers on Feb. 12, 1935 entitled SHAFTING FOR POWER TRANSMISSION teaches shafting structures for power transmission to balance forces in relation to airplanes.
U.S. Pat. No. 3,519,227 issued to Brooks on Jul. 7, 1970 entitled MOUNTING MEANS FOR AN AIRCRAFT ENGINE teaches an engine containing pod mounted from an aircraft structure and at least one fence extending from the pod or the aircraft structure adjacent the pod. The fences are slightly curved.
U.S. Pat. No. 5,443,230 issued to Lord et al. on Aug. 22, 1995 entitled AIRCRAFT WING/NACELLE COMBINATION teaches a wing including a pylon with a cambered airfoil which extends toward the fuselage over at least a portion of the pylon to divert a portion of the airflow away from the channel region. The pylon has curvature in both the vertical and horizontal directions. The '230 teaches of a particular pylon construction for the cambered airfoil, which mounts an aircraft engine to the wing of a plane. The pylon is a three dimensional airfoil shape, which has curvature in both the vertical and horizontal directions, in varying degree, in order to increase aerodynamic performance, reduce drag and shockwave effects, and to control airflow entering select channel regions and redirecting the flow outwardly.
U.S. Pat. No. 4,363,630 issued to Di Vigano on Dec. 14, 1982 entitled SHIP'S HULL WITH A HELICAL CHANNEL AHEAD OF EACH PROPELLER AND METHOD OF ESTABLISHING THE SHAPE OF THE SAME teaches a hull design having a partially helicoidal or cochleaform channel leading towards a propeller. The helical channel leads water to the propeller. a ship's hull, which incorporates a helical channel formed in the hull in front of a propeller or multiple propellers. The '630 patent discloses test models, and related hydrodynamic and theoretical analysis related to creating a “dissymmetry” to the hull design. The helical channel is actually incorporated into the ship's hull, and meant to enhance the tangential and axial vector components of the flow of water through the channel incident to the hull. The essence of the patent seeks to impart to the water a macroscopic vortex flow, in order to achieve the desired results.
The prior art fails, however, to provide a particular helical or twisted strut and related structure which matches the rotational inflow to the propeller as opposed to countering it, thus improving the advance velocity into the propeller disk resulting in higher efficiencies.